11,508 research outputs found
Requirements for contractility in disordered cytoskeletal bundles
Actomyosin contractility is essential for biological force generation, and is
well understood in highly organized structures such as striated muscle.
Additionally, actomyosin bundles devoid of this organization are known to
contract both in vivo and in vitro, which cannot be described by standard
muscle models. To narrow down the search for possible contraction mechanisms in
these systems, we investigate their microscopic symmetries. We show that
contractile behavior requires non-identical motors that generate large enough
forces to probe the nonlinear elastic behavior of F-actin. This suggests a role
for filament buckling in the contraction of these bundles, consistent with
recent experimental results on reconstituted actomyosin bundles.Comment: 10 pages, 6 figures; text shortene
Contractile units in disordered actomyosin bundles arise from F-actin buckling
Bundles of filaments and motors are central to contractility in cells. The
classic example is striated muscle, where actomyosin contractility is mediated
by highly organized sarcomeres which act as fundamental contractile units.
However, many contractile bundles in vivo and in vitro lack sarcomeric
organization. Here we propose a model for how contractility can arise in
actomyosin bundles without sarcomeric organization and validate its predictions
with experiments on a reconstituted system. In the model, internal stresses in
frustrated arrangements of motors with diverse velocities cause filaments to
buckle, leading to overall shortening. We describe the onset of buckling in the
presence of stochastic actin-myosin detachment and predict that
buckling-induced contraction occurs in an intermediate range of motor
densities. We then calculate the size of the "contractile units" associated
with this process. Consistent with these results, our reconstituted actomyosin
bundles contract at relatively high motor density, and we observe buckling at
the predicted length scale.Comment: 5 pages, 4 figures, Supporting text and movies attache
Scaling Invariance in a Time-Dependent Elliptical Billiard
We study some dynamical properties of a classical time-dependent elliptical
billiard. We consider periodically moving boundary and collisions between the
particle and the boundary are assumed to be elastic. Our results confirm that
although the static elliptical billiard is an integrable system, after to
introduce time-dependent perturbation on the boundary the unlimited energy
growth is observed. The behaviour of the average velocity is described using
scaling arguments
Quark confinement and color transparency in a gauge-invariant formulation of QCD
We examine a nonlocal interaction that results from expressing the QCD
Hamiltonian entirely in terms of gauge-invariant quark and gluon fields. The
interaction couples one quark color-charge density to another, much as electric
charge densities are coupled to each other by the Coulomb interaction in QED.
In QCD, this nonlocal interaction also couples quark color-charge densities to
gluonic color. We show how the leading part of the interaction between quark
color-charge densities vanishes when the participating quarks are in a color
singlet configuration, and that, for singlet configurations, the residual
interaction weakens as the size of a packet of quarks shrinks. Because of this
effect, color-singlet packets of quarks should experience final state
interactions that increase in strength as these packets expand in size. For the
case of an SU(2) model of QCD based on the {\em ansatz} that the
gauge-invariant gauge field is a hedgehog configuration, we show how the
infinite series that represents the nonlocal interaction between quark
color-charge densities can be evaluated nonperturbatively, without expanding it
term-by-term. We discuss the implications of this model for QCD with SU(3)
color and a gauge-invariant gauge field determined by QCD dynamics.Comment: Revtex, 23 pages; contains additional references with brief comments
on sam
The Prediction of Mass of Z'-Boson from Mixing
B_q^0-B_^0 bar mixing offers a profound probe into the effects of new
physics beyond the Standard Model. In this paper, and
mass differences are considered taking the effect of both
Z-and Z' -mediated flavour-changing neutral currents in the
mixing (q = d, s). Our estimated mass of Z' boson is accessible at the
experiments LHC and B-factories in near future.Comment: 11 pages, 02 Figure
Stopping Light All-Optically
We show that light pulses can be stopped and stored all-optically, with a
process that involves an adiabatic and reversible pulse bandwidth compression
occurring entirely in the optical domain. Such a process overcomes the
fundamental bandwidth-delay constraint in optics, and can generate arbitrarily
small group velocities for light pulses with a given bandwidth, without the use
of any coherent or resonant light-matter interactions. We exhibit this process
in optical resonator systems, where the pulse bandwidth compression is
accomplished only by small refractive index modulations performed at moderate
speeds. (Accepted for publication in Phys. Rev. Lett. Submitted on Sept. 10th
2003)Comment: 18 pages including 3 figures. Accepted for publication in Phys. Rev.
Let
Quantum Electrodynamics in the Light-Front Weyl Gauge
We examine QED(3+1) quantised in the `front form' with finite `volume'
regularisation, namely in Discretised Light-Cone Quantisation. Instead of the
light-cone or Coulomb gauges, we impose the light-front Weyl gauge . The
Dirac method is used to arrive at the quantum commutation relations for the
independent variables. We apply `quantum mechanical gauge fixing' to implement
Gau{\ss}' law, and derive the physical Hamiltonian in terms of unconstrained
variables. As in the instant form, this Hamiltonian is invariant under global
residual gauge transformations, namely displacements. On the light-cone the
symmetry manifests itself quite differently.Comment: LaTeX file, 30 pages (A4 size), no figures. Submitted to Physical
review D. January 18, 1996. Originally posted, erroneously, with missing
`Weyl' in title. Otherwise, paper is identica
Extinctions and Correlations for Uniformly Discrete Point Processes with Pure Point Dynamical Spectra
The paper investigates how correlations can completely specify a uniformly
discrete point process. The setting is that of uniformly discrete point sets in
real space for which the corresponding dynamical hull is ergodic. The first
result is that all of the essential physical information in such a system is
derivable from its -point correlations, . If the system is
pure point diffractive an upper bound on the number of correlations required
can be derived from the cycle structure of a graph formed from the dynamical
and Bragg spectra. In particular, if the diffraction has no extinctions, then
the 2 and 3 point correlations contain all the relevant information.Comment: 16 page
Quantum Mechanics of the Vacuum State in Two-Dimensional QCD with Adjoint Fermions
A study of two-dimensional QCD on a spatial circle with Majorana fermions in
the adjoint representation of the gauge groups SU(2) and SU(3) has been
performed. The main emphasis is put on the symmetry properties related to the
homotopically non-trivial gauge transformations and the discrete axial symmetry
of this model. Within a gauge fixed canonical framework, the delicate interplay
of topology on the one hand and Jacobians and boundary conditions arising in
the course of resolving Gauss's law on the other hand is exhibited. As a
result, a consistent description of the residual gauge symmetry (for
SU(N)) and the ``axial anomaly" emerges. For illustrative purposes, the vacuum
of the model is determined analytically in the limit of a small circle. There,
the Born-Oppenheimer approximation is justified and reduces the vacuum problem
to simple quantum mechanics. The issue of fermion condensates is addressed and
residual discrepancies with other approaches are pointed out.Comment: 44 pages; for hardcopies of figures, contact
[email protected]
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